The purpose of these studies is to accurately document possible racial, environmental and geographic differentials in the prevalence of major disorders by surveying an entire geographically defined population. The disorders investigated include cerebral palsy, dementia, psychomotor delay, epilepsy, Parkinson's disease, essential tremor, multiple sclerosis and cerebrovascular disease. Research protocols for these neurologic studies have been completed in the United States (Copiah County), India and Spain. The prevalence study of the Parsi Community of Bombay, India provided point prevalence estimates of 328.3/100,000 for Parkinson's disease and 1591.7/100,000 for essential tremor. In the pilot prevalence study in an urban area of Madrid, Spain, migraine and epilepsy were the most frequent disorders with point prevalence ratios of 163.3/1000 and 9.3/1000, respectively.